Diesel exhaust fluid delivery system with pressure control

ABSTRACT

A diesel exhaust fluid delivery system and a dual-variable control strategy for adjusting the delivery pressure and the mass flow rate of the fluid passing through the system. The method for operating the system using two operating modes: low flow rate and high flow rate. While operating in the low flow rate operating mode, the speed of a DEF pump remains constant and DEF delivery to the exhaust system is adjusted by controlling a backflow valve. While operating in the high flow rate operating mode, the backflow valve is closed and the speed of the pump is adjusted to regulate DEF delivery to the exhaust system.

RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.62/037,691 filed Aug. 15, 2014, the entire contents of which areincorporated herein by reference.

BACKGROUND

Diesel exhaust fluid (DEF) is an aqueous urea solution that is used tolower NO_(x) concentration in diesel exhaust emissions from dieselengines. The present invention relates to DEF delivery systems andmechanisms for controlling the fluid pressure within the system.

SUMMARY

In order to increase the delivery capability of DEF delivery systemsthat require a return mass flow for operation, an additional valve isintroduced to affect the mass flow balance and thereby increase thetotal delivery of the system. However, because pressure is a variablethat can be used to provide accurate dosage, the addition of a newcomponent (i.e., a backflow valve (BFV)) could increase the complexityof the control mechanism for the system.

With the addition of the BFV, the system adds a new control variabletransforming the DEF delivery system from a SISO (single input, singleoutput) control system to a MISO (multiple input, single output) controlsystem. The systems and methods described below address a multi-variablecontrol problem of a DEF delivery system with a BFV by decoupling bothcontrol variables and making the actuation of each control variableindependent of the other. Additionally, the system robustness in failuredetection is increased as the simultaneous control of the variables isavoided making it easier to determine the possible reasons for systemfailure.

In some embodiments, the system decouples the control variables bydefining two different control windows based on the requested dosingmass flow (i.e., the amount of DEF to be delivered to the exhaustsystem). At mass flow rates below a defined threshold, pressure iscontrolled by a proportional-integral-derivative (PID) controller thatcontrols the actuation of the BFV while the supply pump operates at aconstant delivery speed. At mass flow rates above the defined threshold,pressure is PID controlled by varying actuation of the supply pump whilethe BFV remains fully closed. In some such embodiments, the threshold isdefined as the mass flow rate that would require the BFV to be fullyclosed in order to provide the desired mass flow rate without adjustingthe pump speed.

By operating under two different control schemes depending on the massflow, the determination of the parameters of the PID controllers issimplified as the two control mechanisms do not work against each othersimultaneously. This control strategy keeps the diagnostic strategysimplified as the parallel actuation of both control variables wouldincrease the complexity of pin pointing the reason for undesiredpressure behavior. Unstable pressure (either too high or too low) atlower mass flow rates is an indicator of improper BFV performance.Unstable pressure (i.e., low pressure) at higher mass flow rates is anindicator of improper pump performance.

In one embodiment, the invention provides a diesel exhaust fluiddelivery system that includes a DEF storage tank, a pump, a backflowvalve, at least one dosing module, and a controller. The controller isconfigured to regulate the flow of DEF from the storage tank to thedosing module using a low mass flow strategy when a desired massdelivery flow rate is below a threshold and using a high mass flowstrategy when the desired mass delivery flow rate is above thethreshold. Under the low mass flow strategy, the controller operates thepump at a constant setting regardless of the desired mass delivery flowrate and controllably opens and closes the backflow valve to allow aportion of the diesel exhaust fluid that is pumped from the storage tankby the pump to flow back into the storage tank—thereby reducing thedelivery pressure and achieving the desired mass delivery flow rate.Under the high mass flow strategy, the controller fully closes thebackflow valve and adjusts the setting of the pump to achieve thedesired mass delivery flow rate. In some embodiments, the pump settingthat is adjusted by the controller is pump speed or pump duty cycle.

In another embodiment, the invention provides a method of providingdiesel exhaust fluid from a storage tank through a dosing module at atarget mass delivery flow rate. When the target mass delivery flow rateis below a threshold, the mass delivery flow rate is adjusted toward thetarget mass delivery flow rate by operating a pump at a constantoperating setting and controlling a backflow valve to allow a portion ofthe diesel exhaust fluid that is pumped from the storage tank by thepump to flow back into the storage tank—thereby reducing the deliverypressure and achieving the desired mass delivery flow rate. When thetarget mass delivery flow rate is above the threshold, the backflowvalve is fully closed and the operating setting of the pump is adjustedto achieve the target mass delivery flow rate.

In some embodiments, the threshold between the low flow rate controlstrategy and the high flow rate control strategy is defined as the massdelivery flow rate at which the backflow valve must be fully closed andthe pump must be operated at the constant operating setting in order toachieve the target mass delivery flow rate.

Another embodiment of the invention provides a diesel exhaust fluiddelivery system. The system includes one or more dosing modules, whichare positionable to deliver diesel exhaust fluid to an exhaust system; afluid storage tank, which stores the diesel exhaust fluid used by thesystem; a pump, which is coupled to the fluid storage tank and thedosing modules, and which pumps the diesel exhaust fluid from thestorage tank to the dosing modules; and a backflow valve, which iscoupled to the storage tank, and which controls the amount of the pumpeddiesel exhaust fluid that is allowed to flow back into the storage tankthrough a backflow line. The system also includes a controller, which isconfigured to determine a desired mass flow delivery rate and regulatedelivery of the fluid to the exhaust system. When the desired mass flowdelivery rate is below the threshold, the controller regulates deliveryof diesel exhaust fluid by varying the amount of fluid that is allowedto flow back into the fluid storage tank through the backflow valve.When the desired mass flow delivery rate is above the threshold, thecontroller regulates delivery of diesel exhaust fluid by adjusting aspeed of the pump.

In some embodiments, the system's controller is configured to operatethe pump at a constant defined speed when the desired mass flow deliveryrate is below the threshold.

In some embodiments, the system's controller is configured to fullyclose the backflow valve to prevent pumped diesel exhaust fluid fromflowing back into the storage tank when the desired mass flow deliveryrate is above the threshold.

Some embodiments of the invention include a pressure sensor, which isconfigured to sense the pressure of the diesel exhaust fluid pumped tothe dosing modules. The controller uses the sensed pressure to determinean actual mass flow delivery rate, and it adjusts the delivery of thefluid to the exhaust system to cause the actual mass flow delivery rateto approach the desired mass flow delivery rate.

In some embodiments of the system, the controller determines an actualmass flow delivery rate, and compares the actual mass flow delivery rateto the desired mass flow delivery rate. When the actual mass flowdelivery rate is greater than the desired mass flow delivery rate, andthe desired mass flow delivery rate is below the threshold, thecontroller adjusts the position of the backflow valve to increase theamount of fluid that is allowed to flow back into the fluid storagetank. When the actual mass flow delivery rate is less than the desiredmass flow delivery rate, and the desired mass flow delivery rate isbelow the threshold, the controller adjusts the position of the backflowvalve to decrease the amount of fluid that is allowed to flow back intothe fluid storage tank.

In some embodiments of the system, the controller determines an actualmass flow delivery rate, and compares the actual mass flow delivery rateto the desired mass flow delivery rate. When the actual mass flowdelivery rate is greater than the desired mass flow delivery rate, andthe desired mass flow delivery rate is above the threshold, thecontroller decreases the speed of the pump. When the actual mass flowdelivery rate is less than the desired mass flow delivery rate, and thedesired mass flow delivery rate is above the threshold, the controllerincreases the speed of the pump.

Another embodiment of the invention provides a method for operating adiesel exhaust fluid delivery system. The method includes determining adesired mass flow rate for delivery of diesel exhaust fluid to anexhaust system through one or more dosing modules. When the desired massflow delivery rate is below a threshold, delivery of the fluid to theexhaust system is regulated by varying the amount of fluid that isallowed to flow back into a fluid storage tank through a backflow valve.When the desired mass flow delivery rate is above the threshold,delivery of the fluid to the exhaust system is regulated by adjusting aspeed of a pump configured to pump the fluid from the fluid storage tankto the dosing modules.

In some embodiments, the method also includes operating the pump at aconstant defined speed when the desired mass flow delivery rate is belowthe threshold.

In other embodiments, the method also includes fully closing thebackflow valve to prevent pumped diesel exhaust fluid from flowing backinto the storage tank when the desired mass flow delivery rate is abovethe threshold.

In other embodiments of the method, the method includes receiving thepressure of the diesel exhaust fluid pumped to the dosing modules anddetermining an actual mass flow delivery rate based on the pressure. Thedelivery of the fluid to the exhaust system is then adjusted to causethe actual mass flow delivery rate to approach the desired mass flowdelivery rate.

In some embodiments of the method, the method includes determining anactual mass flow delivery rate and comparing the actual mass flowdelivery rate to the desired mass flow delivery rate. In suchembodiments, where the desired mass flow delivery rate is below thethreshold, the actual mass flow delivery rate is controlled by adjustingthe position of the backflow valve. If the actual mass flow deliveryrate is greater than the desired mass flow delivery rate, then theposition of the backflow valve is adjusted to increase the amount offluid that is allowed to flow back into the fluid storage tank. If theactual mass flow delivery rate is less than the desired mass flowdelivery rate, then the position of the backflow valve is adjusted todecrease the amount of fluid that is allowed to flow back into the fluidstorage tank.

In other embodiments of the method, the method includes determining anactual mass flow delivery rate and comparing the actual mass flowdelivery rate to the desired mass flow delivery rate. In suchembodiments, where the desired mass flow delivery rate is above thethreshold, the actual mass flow delivery rate is controlled by adjustingthe speed of the pump. If the actual mass flow delivery rate is greaterthan the desired mass flow delivery rate, then the speed of the pump isdecreased. If the actual mass flow delivery rate is less than thedesired mass flow delivery rate, then the speed of the pump isincreased.

Other aspects of the invention will become apparent by consideration ofthe detailed description and accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic drawing of a diesel exhaust fluid delivery systemaccording to one embodiment.

FIG. 2 is a block diagram of various electrical variables that affectthe back flow valve mass flow in the system of FIG. 1.

FIG. 3 is a block diagram of various hydraulic variables that affect theback flow valve mass flow in the system of FIG. 1.

FIG. 4 is a graph of BFV mass flow as a function of the BFV actuationduty cycle at 20 degrees C.

FIG. 5 is a flow chart of a method of controlling the system of FIG. 1according to a dual-variable control mechanism.

FIG. 6 is a graph of pump speed and backflow valve duty cycle as afunction of dosing quantity for the system of FIG. 1 operating accordingto the method of FIG. 5.

FIG. 7 is a graph of experimental results of various pressure valueswithin the system of FIG. 1 as a function of time while operating at −5degrees C.

FIG. 8 is a graph of experimental results of various pressure valueswithin the system of FIG. 1 as a function of time while operating at 60degrees C.

FIG. 9 is a graph of experimental results of various pressure valueswithin the system of FIG. 1 as a function of time while operating acrosstransient and variable temperatures.

DETAILED DESCRIPTION

Before any embodiments of the invention are explained in detail, it isto be understood that the invention is not limited in its application tothe details of construction and the arrangement of components set forthin the following description or illustrated in the following drawings.The invention is capable of other embodiments and of being practiced orof being carried out in various ways.

It should also be noted that a plurality of hardware and software baseddevices, as well as a plurality of different structural components maybe used to implement the invention. In addition, it should be understoodthat embodiments of the invention may include hardware, software, andelectronic components or modules that, for purposes of discussion, maybe illustrated and described as if the majority of the components wereimplemented solely in hardware. However, one of ordinary skill in theart, and based on a reading of this detailed description, wouldrecognize that, in at least one embodiment, the electronic based aspectsof the invention may be implemented in software (e.g., stored onnon-transitory computer-readable medium) executable by one or moreprocessors. As such, it should be noted that a plurality of hardware andsoftware based devices, as well as a plurality of different structuralcomponents may be utilized to implement the invention. For example,“control units” and “controllers” described in the specification caninclude one or more processors, one or more memory modules includingnon-transitory computer-readable medium, one or more input/outputinterfaces, and various connections (e.g., a system bus) connecting thecomponents.

FIG. 1 illustrates an example of a DEF delivery system 10. The systemincludes one or more dosing modules 11 for delivering DEF to an exhaustsystem and a supply module 12 for moving DEF from a storage tank 13 tothe dosing modules 11 at a desired, variable pressure or mass deliveryflow rate. The supply module 12 includes a pump 14, a reverting valve15, a main filter 16, and a controllable backflow valve (BFV) 17. Thepump 14 draws DEF from the storage tank 13 through a suction line 18 andprovides the DEF to the dosing modules 11 through a pressure line 19.The backflow valve 17 controllably allows some of the pumped DEF to flowback into the storage tank 13 through backflow line 20 thereby reducingthe pressure of the DEF in the pressure line 19. The supply module 12also includes a pressure sensor 21 and a controller 22 capable ofimplementing PID control for the pump 14 and the BFV 17. The controller22 and the pressure sensor 21 regulate the pressure within the pressureline 19 as described in further detail below. Also, by using aproportional valve as a backflow valve 17, it is possible to adapt thebackflow continuously to the dosing amount to hold the overall flow rateconstant over a wide range.

In some embodiments, the controller 22 includes a plurality ofelectrical and electronic components that provide power, operationalcontrol, and protection to the components and modules within thecontroller 22. The controller 22 includes, among other things, aprocessing unit (e.g., a microprocessor or another suitable programmabledevice), a memory, and an input/output interface. The processing unit,the memory, and the input/output interface, as well as the other variousmodules are connected by one or more control or data buses. The use ofcontrol and data buses for the interconnection between and communicationamong the various modules and components would be known to a personskilled in the art in view of the invention described herein. In someembodiments, the controller 22 is implemented partially or entirely on asemiconductor (e.g., a field-programmable gate array [“FPGA”]semiconductor) chip.

The memory includes a program storage area and a data storage area. Theprogram storage area and the data storage area can include combinationsof different types of memory, such as read-only memory (“ROM”), randomaccess memory (“RAM”) (e.g., dynamic RAM (“DRAM”), synchronous DRAM(“SDRAM”), etc.), electrically erasable programmable read-only memory(“EEPROM”), flash memory, a hard disk, an SD card, or other suitablemagnetic, optical, physical, or electronic memory devices. Theprocessing unit is connected to the memory and executes softwareinstructions that are capable of being stored in a RAM of the memory(e.g., during execution), a ROM of the memory (e.g., on a generallypermanent basis), or another non-transitory computer readable medium.Software included for the processes and methods for the DEF deliverysystem 10 can be stored in the memory of the controller 22. The softwarecan include firmware, one or more applications, program data, filters,rules, one or more program modules, and other executable instructions.For example, the controller 22 effectively stores information relatingto the pressure control of DEF delivery system 10. The processing unitis configured to retrieve from memory and execute, among other things,instructions related to the control processes and methods describedherein. In other constructions, the controller 22 includes additional,fewer, or different components.

In this system, the mass flow balance is defined such that the sum ofthe mass flow rate through the backflow line 20 and the mass flow ratethrough the pressure line 19 is equal to the mass flow rate through thesuction line 18 (i.e., m_(backflow)+m_(pressure)=m_(suction)). As such,when operating at a constant suction mass flow rate, the mass flow ratethrough the backflow line 20 must decrease in order to increase the massflow rate through the pressure line 19 and vice versa.

There are a large number of electrical and hydraulic variables that canaffect the rate of mass flow through the backflow valve 17. FIGS. 2 and3 illustrate some examples of such variables. BFV mass flow 30 isdirectly affected by actuation current 32 (i.e., electrical variables)and valve mechanics 34 (i.e., hydraulic/mechanical variables.

FIG. 2 illustrates some of the electrical variables that can affect theactuation current 32 in the DEF delivery system 10. The overallresistance 36 of the BFV system, the battery voltage 38, and theactuation duty cycle 40 of the BFV 17 all affect the actuation current32. Power stage resistance 42, wiring harness resistance 44, crimpresistance 46, and coil resistance 48 all affect the overall resistance36. The resistance of each of these components can, however, varydepending upon temperature 50. Particularly, the coil resistance 48 isdependent on ambient temperature (T_(amb)) 52, the temperature of theDEF (TDEF) 54, the temperature of system coolant (T_(coolant)) 56, andtemperature increases due to power losses.

FIG. 3 illustrates some of the hydraulic variables that can affect thevalve mechanics 34 in the DEF delivery system 10, which, as noted above,directly affects the BFV mass flow rate. Valve mechanics 34 are affectedby production tolerances 58 of the BFV 17, adjustment screw position 60,valve seat material 62, orifice size 64, valve spring 66, and pressure68 of the DEF in the pressure line 19. Again, the actuation current 32and the valve mechanics 34 both affect the mass flow rate of the BFV 17.

The variance in actual operation due to these variables is illustratedin FIG. 4. FIG. 4 shows the actual mass flow rate as a function of theBFV actuation duty cycle for a variety of samples. The DEF deliverysystem 10 is operating at an ambient temperature of 20° C. and at 28.5 Vin each sample. However, there is demonstrated variation in the measuredmass flow rate across the samples.

FIG. 5 illustrates a method of operating the DEF delivery system of FIG.1 using a dual-variable control mechanism. This method addresses theconcerns of BFV mass flow rate fluctuations and provides predictable andreliable mass flow delivery through the dosing modules 11. The methodprovides for two operating modes: low flow rate and high flow rate.While operating in the low flow rate operating mode, the speed/dutycycle of the pump 14 remains constant and the pressure of the DEF in thepressure line 19 is adjusted by controlling the mass flow returning tothe storage tank 13 through the BFV 17. While operating in the high flowrate operating mode, the BFV 17 is fully closed and the speed of pump 14is adjusted/controlled to regulate the pressure of the DEF in thepressure line 19.

In step S1, the controller 22 determines the desired mass flow rate. Instep S2, the desired mass flow rate is compared to a threshold. If thedesired mass flow rate is less than a threshold, then the system isoperated in a low flow rate operating mode. In step S3, the pump 14 isset to a constant duty cycle or speed (or other operating parameter). Instep S4, the actual mass flow rate is compared to the desired mass flowrate. If the actual mass flow rate is greater than the desired mass flowrate, the BFV 17 is adjusted in step S5 to allow greater mass flow ofDEF through BFV 17 and backflow line 20 into the storage tank 13. Thisincrease in backflow of DEF reduces the pressure of the DEF in thepressure line 19, thereby reducing the actual mass flow rate. If,however, the actual mass flow rate is less than the desired mass flowrate, the BFV 17 is adjusted in step S6 to decrease the amount of DEFthat is allowed to flow through the BFV 17 and backflow line 20 into thestorage tank 13. This decrease in backflow of DEF increases the pressureof the DEF in the pressure line 19, thereby increasing the actual massflow rate. Thus, the method provides that while operating in the lowflow rate operating mode, the speed/duty cycle of the pump 14 remainsconstant and the pressure of the DEF in the pressure line 19 is adjustedby controlling the mass flow through the BFV 17.

If, however, in step S2, the desired mass flow rate is greater than thethreshold, the system is operated in a high flow rate operating mode. Instep S7, the BFV 17 is fully closed. In step S8, the actual mass flowrate is compared to the desired mass flow rate. If the actual mass flowrate is less than the desired mass flow rate, then the speed of pump 14is increased in step S10, which increases the pressure of the DEF in thepressure line 19, thereby increasing the actual mass flow rate. If theactual mass flow rate is greater than the desired mass flow rate, thenspeed of pump 14 is decreased in step S9, which decreases the pressureof the DEF in the pressure line 19, thereby decreasing the actual massflow rate.

FIG. 6 illustrates the pump speed 70 and BFV duty cycle 72 as a functionof the dosing quantity (desired mass flow rate) according to the methodof FIG. 5. The pump speed 70 is shown as a percentage of the maximumpump speed. The BFV duty cycle 72 represents the position of the valveas a percentage of “openness,” with 100% corresponding to a fully openvalve, and 0% corresponding to a fully closed valve. When the dosingquantity is below the threshold 74 and the system 10 is operating in thelow flow rate operating mode (BFV control mode), the pump speed 70remains constant while the BFV duty cycle 72 decreases as the desiredmass flow rate increases. When the dosing quantity is above thethreshold 74 and the system 10 is operating in the high flow rateoperating mode (pump control mode), the BFV valve is fully closed,decreasing BFV flow rate 72 to zero, while the pump speed 70 isincreased as the desired mass flow rate increases.

The threshold can be defined based on operational preferences and systemdesign. The threshold may be defined as a constant value—for example, inFIG. 6, the threshold is set at 12 kg/h. Alternatively, the system 10may implement a varying threshold that is adjusted based on operatingconditions (e.g., temperature). In some systems, the threshold may bedefined based on the mass flow rate that would generally be achieved ifthe BFV were placed in the fully closed position and the pump wereoperated at the constant speed of the “low flow rate” operating mode.

The system 10 may also implement a floating threshold. For example, thesystem 10 may be configured to switch from the “low flow rate” operatingmode to the “high flow rate” operating mode when the desired mass flowrate reaches a level where the control mechanism can no longer raise themass flow rate by adjusting the BFV (i.e., the BFV is already fullyclosed). Similarly, the system 10 may be configured to switch from the“high flow rate” operating mode to the “low flow rate” operating modewhen a high flow rate control mechanism would dictate that the pump beoperated below a define speed threshold.

At the transition point from one operating mode to the other, theintegrator portions of the PID controls for the pump 14 and the BFV 17are reset to ensure that the transition between modes goes smoothly anddoes not significantly influence the pressure in the system 10. Thereset values depend on the sensitivity of the actuator being controlled.(In this case, sensitivity is a measure of the rate of change of theoutput for a given input: the higher change in mass flow for a givenchange in actuation duty cycle, the higher the sensitivity.) The pump 14in system 10 is not highly sensitive, therefore the reset value for thePID controlling the pump 14 can be reset to the expected pump duty cycleof a nominal part, which is based on the actual dosing request. However,a fixed reset value cannot be determined for the BFV 17 because of thewide range of duty cycle values that the BFV can take based on theboundary conditions, as shown in FIG. 2. In addition, the BFV 17 is thefastest actuator in system 10, and it has a high sensitivity.Accordingly, the controller 22 must accurately determine a reset valuefor the BFV 17. The controller 22 determines the value by learning itduring certain portions of the operation of the BFV 17. The value islearned when the following conditions exist for a period of timesufficient to learn the value: the dosing request is at or near zero,the system is operating to meter the mass flow, and the pressure iswithin a range of 8900 and 9100 mbar. The reset value for the PIDcontroller is updated whenever the conditions are met. BFV 17 dutycycles for different mass flow dosing requests can be determined usingthe learned duty cycle reset value.

The closed loop system illustrated in FIGS. 5 and 6 reduces oreliminates the temperature dependency of the control system.Furthermore, BFV production tolerances are compensated for. FIGS. 7, 8,and 9 illustrate various experimental data from the system of FIG. 1using the control mechanism of FIGS. 5 and 6. The top graph in eachfigure illustrates the pressure in the pressure line 19 and the dosingrequest (desired mass flow rate) as a function of time. The middle graphin each figure shows the speed of pump 14 (Pump DC) as demonstrated bythe drawing pressure of the pump in hPa and the flow rate of the BFV 17(BFV DC) in kg/h both as functions of time. The bottom graph in eachfigure shows the temperature of the DEF in the storage tank 13 and theambient temperature during system operation as a function of time. InFIG. 7, the system is operated at −5° C. In FIG. 8, the system isoperated at an ambient temperature of approximately 60° C. In FIG. 9,the ambient temperature is varied between 50° C. and 0° C.

Thus, the invention provides, among other things, a diesel exhaust fluiddelivery system and a dual-variable control strategy for adjusting thedelivery pressure and the mass flow rate of the fluid passing throughthe system. Various features and advantages of the invention are setforth in the accompanying drawings.

What is claimed is:
 1. A diesel exhaust fluid delivery system, thesystem comprising: one or more dosing modules positionable to deliver afluid to an exhaust system; a fluid storage tank; a pump coupled to thefluid storage tank and the one or more dosing modules to pump the fluidfrom the storage tank to the one or more dosing modules; a backflowvalve coupled to the storage tank to controllably allow an amount of thepumped fluid to flow back into the storage tank through a backflow line;an electronic controller configured to determine a target mass flowdelivery rate, regulate delivery of the fluid to the exhaust system byvarying the amount of fluid that is allowed to flow back, via thebackflow valve, into the fluid storage tank through the backflow valvewhen the target mass flow delivery rate is below a threshold, andregulate delivery of the fluid to the exhaust system by adjusting aspeed of the pump when the target mass flow delivery rate is above thethreshold.
 2. The diesel exhaust fluid delivery system of claim 1,wherein the electronic controller is further configured to operate thepump at a constant defined speed when the target mass flow delivery rateis below the threshold.
 3. The diesel exhaust fluid delivery system ofclaim 1, wherein the electronic controller is further configured tofully close the backflow valve and prevent pumped fluid from flowingback into the storage tank when the target mass flow delivery rate isabove the threshold.
 4. The diesel exhaust fluid delivery system ofclaim 1, further comprising a pressure sensor configured to sense apressure of the fluid pumped to the one or more dosing modules, whereinthe electronic controller is further configured to regulate delivery ofthe fluid to the exhaust system by determining an actual mass flowdelivery rate based on a signal from the pressure sensor, and adjustingthe delivery of the fluid to the exhaust system to cause the actual massflow delivery rate to approach the target mass flow delivery rate. 5.The diesel exhaust fluid delivery system of claim 1, wherein theelectronic controller is further configured to: determine an actual massflow delivery rate, compare the actual mass flow delivery rate to thetarget mass flow delivery rate, adjust a position of the backflow valveto increase the amount of fluid that is allowed to flow back into thefluid storage tank when the actual mass flow delivery rate is greaterthan the target mass flow delivery rate and the target mass flowdelivery rate is below the threshold, and adjust the position of thebackflow valve to decrease the amount of fluid that is allowed to flowback into the fluid storage tank when the actual mass flow delivery rateis less than the target mass flow delivery rate and the target mass flowdelivery rate is below the threshold.
 6. The diesel exhaust fluiddelivery system of claim 1, wherein the electronic controller is furtherconfigured to: determine an actual mass flow delivery rate, compare theactual mass flow delivery rate to the target mass flow delivery rate,decrease the speed of the pump when the actual mass flow delivery rateis greater than the target mass flow delivery rate and the target massflow delivery rate is above the threshold, and increase the speed of thepump when the actual mass flow delivery rate is less than the targetmass flow delivery rate and the target mass flow delivery rate is abovethe threshold.
 7. A method for operating a diesel exhaust fluid deliverysystem, the method comprising: using an electronic controller todetermine a target mass flow rate for delivery of a fluid to an exhaustsystem through one or more dosing modules, regulating delivery of thefluid to the exhaust system by varying the amount of fluid that isallowed to flow back into a fluid storage tank through a backflow valvewhen the target mass flow delivery rate is below a threshold, andregulating delivery of the fluid to the exhaust system by adjusting aspeed of a pump configured to pump the fluid from the fluid storage tankto the one or more dosing modules when the target mass flow deliveryrate is above the threshold.
 8. The method of claim 7, furthercomprising: operating the pump at a constant defined speed when thetarget mass flow delivery rate is below the threshold.
 9. The method ofclaim 7, further comprising: fully closing the backflow valve andprevent pumped fluid from flowing back into the storage tank when thetarget mass flow delivery rate is above the threshold.
 10. The method ofclaim 7, further comprising: receiving a pressure of the fluid pumped tothe one or more dosing modules, determining an actual mass flow deliveryrate based on the pressure, and adjusting the delivery of the fluid tothe exhaust system to cause the actual mass flow delivery rate toapproach the target mass flow delivery rate.
 11. The method of claim 7,further comprising: determining an actual mass flow delivery rate,comparing the actual mass flow delivery rate to the target mass flowdelivery rate, adjusting a position of the backflow valve to increasethe amount of fluid that is allowed to flow back into the fluid storagetank when the actual mass flow delivery rate is greater than the targetmass flow delivery rate and the target mass flow delivery rate is belowthe threshold, and adjusting the position of the backflow valve todecrease the amount of fluid that is allowed to flow back into the fluidstorage tank when the actual mass flow delivery rate is less than thetarget mass flow delivery rate and the target mass flow delivery rate isbelow the threshold.
 12. The method of claim 7, further comprising:determining an actual mass flow delivery rate, comparing the actual massflow delivery rate to the target mass flow delivery rate, decreasing thespeed of the pump when the actual mass flow delivery rate is greaterthan the target mass flow delivery rate and the target mass flowdelivery rate is above the threshold, and increasing the speed of thepump when the actual mass flow delivery rate is less than the targetmass flow delivery rate and the target mass flow delivery rate is abovethe threshold.